Hello @JamieKay, since sudo is not possible in the build-image, you won’t be able to use apt-get. You do have access to the internet so you can download whatever dependency you need as a static binary and call it directly (without installing). Alternatively, you can file an issue here if you feel like your dependency should be included in the image by default.

You can grab files from anywhere on the internet during build as long as it is accessible to the public (since we don’t work from a “steady” pool of IP addresses during build). Another pattern you could use is to pre-build a binary and check it into your repo, and use it directly. Make sure you target the appropriate OS and architecture for our build network for this binary (see https://github.com/netlify/build-image#running-locally for more details; currently ubuntu 14.04 or 16.04 x86_64 architecture is used during builds).